Automatic transmissions for both two-wheel drive and four-wheel drive vehicles may have a closed-barrel design wherein automatic transmission fluid (e.g., oil) is circulated through a hydraulic control circuit. Oil may be pumped into the upper case for circulation through valve body components, clutches, and lubrication circuits, and returned by means of drainback openings in the bottom of the upper case. Accumulation of oil in the upper case may increase contact between the oil and rotating components of the transmission. This increased contact may increase parasitic losses in the transmission. Furthermore, accumulation of oil in the upper case may exacerbate problems relating to maintaining an adequate reservoir of oil for the oil pump.